Industrial boilers heat up highly purified feedwater to generate steam for power generation, heating, etc.
A natural consequence of steam production is the "cycling up" in concentration of chemicals which enter the boiler inadvertently (e.g., acid leaks) or intentionally (e.g., corrosion inhibitors). A small portion of the boiler water is "blown down" (i.e., removal of concentrated boiler water from the boiler) to keep the concentrations of non-volatile chemicals (i.e., chemicals that do not flow out with the steam but rather remain substantially in the boiler water) at acceptable levels. The rate of blowdown is defined by the "cycles of concentration." The term "cycles of concentration" is defined as the sum of the steam and blowdown flowrates divided by the blowdown flowrate. Cycles in high pressure boilers range from less than 10 to 100 or more. Thus a chemical added at a low concentration (e.g., 0.5 ppm) in the feedwater can cycle up to fairly high boiler concentrations (e.g., 30 ppm).
These boilers are susceptible to, among other things, corrosion. To minimize corrosion, one basic type of corrosion control program that is practiced in the United States within these boilers is phosphate control programs. Typically, in phosphate control programs, a sodium phosphate salt is fed into the solution in order to buffer the solution and to maintain that pH with sodium. The objective of these phosphate control programs is to maintain the measured variables, phosphate and pH, within certain stated guidelines, which are dependent upon boiler pressure by controlling the sodium, phosphate and resultant pH within the boiler water. See "Sodium Phosphate Solutions at Boiler Conditions: Solubility, Phase Equilibria, and Interactions with Magnetite," by G. Economy, A. J. Panson, Chia-tsun Liu, J. N. Esposito, and W. T. Lindsay, Jr., Proc. Intl. Water Conf. 1975, pp. 161-173.
If the concentration of sodium within the boiler water (which is given by the pH, i.e., pH is proportional to effective sodium) is divided by the concentration of phosphate within the boiler water, there exists a range of optimum sodium-to-phosphate (Na/PO.sub.4) ratios that, if achieved and maintained within the boiler water, will minimize corrosion. Where the boiler water is operated and maintained at a Na/PO.sub.4 ratio that is below 3.0:1, the boiler is said to be operating with coordinated phosphate/pH control (also known as "captive alkalinity"). Where the boiler water is operated and maintained at a Na/PO.sub.4 ratio that is between 2.2:1 and 2.8:1, the boiler is said to be operating with congruent control. Where the boiler is water is operated at a Na/PO.sub.4 ratio that is above 3.0, a boiler is said to be operating with "equilibrium phosphate control." All three types of control can be attained and maintained with the instant invention.
With any of these corrosion control programs, the boiler uses phosphate as the major buffering agent. Additionally, sodium and phosphate concentrations are interdependent variables that must either be controlled simultaneously, or one subservient to the other. They cannot be controlled independently.
Furthermore, boiler systems are extremely slow systems because they comprise large volumes. As an example, a 280,000 pound water boiler having a blowdown rate of 3000 pounds/hour takes over three days to remove and replenish the boiler water. Many things can happen during that time that can alter the operator's initial guess at what concentrations should be added to manually correct control problems.
The applicants have found that conventional control schemes like Proportional Integral Derivative (PID) control are insufficient to provide practical, universally applicable automatic control of this boiler chemistry for a number of reasons. First, setpoint overshoot is a problem when attempting to control pH in a large volume system. Limitations in pumping capacity inherent in a real-life pumping scheme make "integral windup" a serious problem. Integral windup causes a control system to overshoot its setpoint. Overshoot is also a problem in controlling pH with PID control due to the asymmetric nature of pH control. Although this problem could potentially be avoided using blowdown flow controllers, these devices are expensive and difficult to maintain and calibrate.
Second, tuning such a PID loop is very difficult. Although tuning can be done in many ways, the methods generally require one of two sets of conditions be maintained, either of which are difficult to achieve in an operating boiler. In one general tuning method, the boiler chemistry must be held constant for multiples of the first order time constant defined by the volume of the boiler divided by its blowdown flow rate. In real-life applications, such a steady-state cannot be established for that length of time due to small perturbations in feedwater contaminants concentrations. In the other general tuning method, the boiler chemistry must be driven out of the region normally considered to be non-corrosive to derive the tuning constants. This negates the beneficial effect of the treatment. Since any change in blowdown flow rate (a normal part of boiler operations) will render the measured tuning constant invalid, tuning must be repeated for each blowdown flow setting.
There is one reference to sodium/phosphate control in the literature which demonstrates the difficulties of this method and its shortcomings. In "A Practical Approach to Real Time Data Acquisition and Automated Chemical Feed at a Fossil Fueled Cycling Duty Station", by C. E. Frederick presented at the International Conference on Cycle Chemistry in Fossil Plants, Jun. 4-6, 1991, the boiler system was tuned using a semi-empirical method to a specific boiler, rather than being adaptable to various types and sizes of industrial boilers. Furthermore, the system disclosed in that reference requires the use of phosphate analyzers which are expensive and require frequent re-calibrations and maintenance.
The closest art to automatically controlling the Na/PO.sub.4 ratio in the water of an industrial boiler is in automated pH control systems. The control of pH is in itself a difficult task, as discussed in U.S. Pat. No. 5,132,916 (Gulaian et al.).
The following U.S. Patents disclose examples of automated pH control systems: U.S. Pat. No. 4,053,743 (Niemi), U.S. Pat. No. 4,239,493 (Niemi et al.), U.S. Pat. No. 4,181,951 (Boeke), U.S. Pat. No. 5,132,916 (Gulaian), U.S. Pat. No. 5,262,963 (Stana), U.S. Pat. No. 4,016,079 (Severin), U.S. Pat. No. 5,248,577 (Jerome), U.S. Pat. No. 4,033,871 (Wall) and U.S. Pat. No. 5,057,229 (Schulenberg).
The Niemi patent discloses an automatic system for controlling the pH and other concentration variables in a chemical reactor. However, use of that system would not be adaptable to an industrial boiler for the following reasons. The system utilizes a method that requires a steady state that is reached rapidly, which, as discussed previously, an industrial boiler does not exhibit. Consequently, the Niemi patent teaches controlling pH by use of a PID controller, which, as discussed previously, would be difficult to use in Corrosion Control Phosphate (CCP) programs described above.
The Niemi et al. patent discloses an automatic system for controlling the pH in a continuous flow vessel. However, this system is also not adaptable to industrial boilers for the following reasons. For boiler systems, the known tuning methods do not apply for the reasons described above. If the residence time distribution is known, then simulation of tuning methods requires a perfect match of a simulator and reality. The assumptions of linear processes of first order reactions is not applicable. Therefore, the method listed in Niemi et al. will only work for systems with small perturbations. Industrial boilers exhibit larger deviations. Furthermore, Niemi et al. identifies proportional, proportional-integral and proportional-integral-derivative controls along with an adjustable gain controller. Limitations on feed concentrations versus system volume will make any adjustable gain ineffective when bounded by limitations in a "pumpable region." Finally, the same pumpable limitations will make integral windup a serious problem in a large volume system.
The Boeke patent discloses an automatic control system for the adjustment of pH that is described using the term "on-off". However, this is not an ON/OFF controller. The series of solenoids that actuate flow across different size orifices produce a signal proportional to feedback. The series of solenoids provides proportional response that is discreet within a specific flow window. This is analogous to a stepwise integration of a continuous function.
The Gulaian patent discloses an automatic system for controlling pH and utilizing an estimation for a pH titration curve in the adaptive control of pH. However, this system is also relegated to short residence times and the use of proportional-integral control. Furthermore, the patent does not discuss limitations from integral windup.
The Stana patent discloses an automatic system for controlling a phosphoric acid plant. However, this system does not involve a model of the system but rather teaches a target feed where the system is compensated for its chemical deficit and then placed in steady state. The algorithm utilized by the system contains predetermined constants that are unique to a particular phosphoric acid plant, and are therefore, not readily adaptable to a variety of phosphoric acid plants (e.g., different plant volumes would require that new constants be calculated and inserted into the algorithm). Moreover, this system controls only sulfuric acid feed and does not try to control two interdependent variables.
The Severin patent discloses an automatic chlorine and pH control apparatus for swimming pools. The apparatus controls two variables, i.e., chlorine and pH, under the assumption that the two are not interrelated. Although chlorine affects pH, chlorine has a minor effect on pH and can be isolated and controlled separately. This is because in a swimming pool, chlorine is not the only buffering agent. Its contribution to the pH is masked by the high concentration of anions from the makeup water and atmosphere. This allows the pH to be controlled independent of the chlorine concentration. In contrast, as discussed earlier, a congruent controlled boiler uses phosphate as the major buffering agent, and the pH and phosphate are interdependent variables that must either be controlled simultaneously, or one subservient to the other. They cannot be controlled independently. In addition, the Severin apparatus also ignores the cycle time of a swimming pool and assumes that the control is constant through the system. It does not account for lag and residence time effects and probably cycles up and down drastically when in operation. Finally, the pH control range is anticipated as narrow, and works on the assumption that pH is linear in the chosen range.
The Wall patent discloses a system for continuously monitoring and controlling the pH and free halogen in swimming pool water. Although this patent mentions the concept of two-sided control (i.e., monitoring whether pH or halogen or both fall within or without predetermined ranges), the control of the pH of swimming pools and the control of pH in a boiler are not interchangeable, as described above.
The Schulenberg patent discloses an automatic system treatment of cooling circuit water. Although this system describes on/off pH control of a single component to provide one sided control and the system adds other components according to vaporous loss, the chemistry is different from that of a boiler. The chlorine in the Schulenberg patent is not the major buffer, and no attempt is made to maintain the CO.sub.2 alkalinity. In addition, this system cannot control two interdependent variables. The corrosion inhibitor and the pH are not interdependent as are the phosphate (similar to a corrosion inhibitor) and pH.
The Jerome patent discloses a reactant concentration control method and apparatus for precipitation reactions. The system does base feed one reagent and adjusts the second. However, the method and apparatus assume that the system is near steady state at all times. The calculations are linearized and performed incrementally to make the calculations simpler. The model used in the method/apparatus is not a true continuously stirred tank reactor (as is the model for industrial boilers).
U.S. Pat. No. 5,141,716 (Muccitelli), which is owned by the same Assignee of the present patent application discloses a method of reducing corrosion in a boiler using coordinated phosphate control. However, this method calls for the administering of particular hydroxyethyl piperazines in specific ratios with phosphate, i.e., there is no automatic apparatus nor methods disclosed of conducting this feed.
Two other references which discuss coordinated phosphate control are: Justification and Engineering Design for the On-Line Monitoring and Automation of a Congruent Phosphate/pH Program by Michael E. Rogers, Ian Verhappen and Stephen Porter, Paper No. 413, The NACE Annual Conference and Corrosion Show 1992; Expert System Helps Fine-Tune Boiler-Water Chemistry, by Leyon O. Bretsel and Lon C. Brouse, Power Magazine 1987. In the former reference, although a proposal is discussed for controlling phosphate feed to the feedwater while controlling conductivity in the boiler water, there is no disclosure of any automatic simultaneous control of phosphate and congruency (Na/PO.sub.4 ratio). With regard to the latter reference, although there is a discussion of providing the operator with chemical feed adjustments, there is no real-time, automatic control system that is disclosed for controlling the chemical pumps in order to control congruency.
Therefore the prior art does not disclose an effective method for controlling two interdependent and non-volatile chemicals, e.g., sodium and phosphate, in a multi-boiler system using a common feedwater line whereby each of the boilers are rarely at steady state. None of the above cited art have devised an apparatus nor a method for achieving an automatic coordinated sodium/phosphate control system for a variety of industrial boilers without the need to introduce separate feed pumps/feed lines for each boiler.